There are presently many different types of plate and fixture systems for securing two or more bones or bone fragments in relative position so that the bones may fuse or heal, or so that tissue adjacent the bones may heal without disruption from the movement of the secured bones. As used herein, the term bone may refer to a bone, or a bone fragment or portion, and the term may refer to a portion of a bone that is covered with another material, such as the endplates covering the top and bottom surface of a vertebra. Also as used herein, the term fusion refers to the joining of materials, such as bone or graft material, and the fusion site is the entire region in which fusion may be desired. As is apparent, bone plate systems may be used to fuse a variety of different types of bone, including spinal vertebrae.
The bones and connective tissues of a normal human spinal column include vertebrae that support the weight of the human body as well as protect the spinal cord and other elements of the nervous system. A typical vertebra consists of a vertebral body and a vertebral arch positioned posterior to the vertebral body. The vertebral body is generally aligned with the vertebral bodies of the adjacent vertebrae to transmit loading forces along the spinal column. The vertebral arch, on the other hand, is formed by a pair of pedicles and a pair of laminae that surround and protect the spinal cord.
The vertebrae of the spine are classified into five regions based on their position along the spinal column: cervical, thoracic, lumbar, sacral, and coccygeal (tail bone). The cervical region contains seven small vertebrae located near the neck and base of the skull, while the thoracic region contains twelve larger vertebrae near the rib cage that increase in size going downward along the spine. The lumbar region is generally in the lower back area and has five vertebrae that are larger than the other vertebrae in the spinal column in order to support the full weight of the upper torso. Normally, the cervical, thoracic, and lumbar vertebrae have intervertebral discs positioned between adjacent vertebral bodies that allow slight movement of the vertebrae and dampen jarring forces which act on the vertebrae.
The sacral and coccygeal vertebrae, on the other hand, are fused in maturity and lack intervertebral discs. More specifically, the sacral region consists of  five vertebrae fused together to form the sacrum, a large triangular bone positioned between the hip bones. The sacrum is relatively concave and faces inward to the body such that the first sacral vertebra (S1) extends obliquely relative to the last lumbar vertebra (L5). Additionally, the anterior-most point of the S1 vertebra includes a formation referred to as the sacral promontory positioned adjacent the intervertebral disc located between the S1 and L5 vertebrae.
Various types of injury or deformity may affect the structure and function of the spinal column. For example, sudden trauma may break bones or tear soft tissues which support the spine. Alternatively, genetic disorders, tumors, or infections may cause bones to deteriorate. Further, excessive movement of one vertebral body relative to another may cause compression of the spinal cord or nerves. Patients who suffer from one or more of these conditions often experience extreme and debilitating pain, and may incur permanent neurologic damage if the conditions are not properly treated.
One technique for treating a number of spinal disorders is known as interbody spinal fusion. This procedure involves removing an intervertebral disc and replacing it with a bone graft in conjunction with stabilizing the vertebrae on either side of the intervertebral disc. The vertebrae eventually fuse together by growth of the bone across the disc space which rigidly connects the vertebrae. The stabilization of the vertebrae is often assisted by a surgically implanted bone plate and anchor system that holds the vertebral bodies in proper alignment and allows the bones to heal. Such techniques have been successfully used to reduce pain and restore the structural integrity of the affected bones.
Different applications of the bone plate may present different requirements for bone plate systems. For example, in some instances it may be desirable to utilize a bone plate system that limits post-operative subsidence of the vertebrae. More specifically, vertebrae may require a higher level of strength and stability to ensure fusion of the affected vertebrae. A bone plate system applied to the vertebrae under these conditions may rigidly fix the bone plate system and the affected vertebrae in the desired orientation to provide the required biomechanical stability while fusion is achieved.
In some instances, it is also highly desirable for a bone plate system to provide a bone anchor that may be driven into bone at a variety of different insertion angles. This functionality provides a surgeon with the flexibility to respond to anticipated or unanticipated features of the subject anatomy. Further, the ability to polyaxially drive the bone anchor into a vertebra also permits the surgeon to angle the bone anchor away from the end plates of the vertebral body. Engaging the bone anchor with the end plate in some instances is undesirable as the bone anchor may cause a weakened end plate to fracture or otherwise reduce the end plate's load bearing capacity.
Many prior bone plate systems fail to provide the desired functionality and ease of use required by a number of bone fusion procedures. One such shortcoming is that many bone plate systems fail to resist backing out or loosening of the bone anchors, which are often bone screws. If the bone screws loosen, the bones are not properly secured and may be allowed to move relative to one another in an uncontrolled manner. This may compromise the ability to achieve optimal bone fusion and bone alignment, and it may lead to loss of graft material and damage or loss of bone. Furthermore, when the plate is located in the lumbar or sacral regions of the spinal column with heavy loading and movement of the vertebrae, these issues may be further compounded or exacerbated. Additionally, in the case of anterior lumbar or anterior sacral plates, a bone screw backing out could cause irritation or a puncture wound to internal organs, or even damage to the circulatory or nervous systems.
Some prior bone plate systems seek to provide polyaxial insertion and fixation of a bone screw using a set screw that expands the bone screw head. In these systems, the bone screw is passed through a bore formed in a bone plate until the bone screw head is positioned within the bore. The set screw is inserted into the bone screw head to expand the head until the head presses against the walls of the bore such that the bone screw is held in place within the bore by the expanded head. However, to permit the bone screw head to expand, the head is constructed to be flexible for this purpose. For example, in U.S. Pat. No. 6,575,975 to Brace et al., the bone screw head is weakened by removing material which permits portions of the head to flex outwardly. When the bone plate is located in the lumbar or sacral region of the spinal column, this approach is undesirable because weakening the bone screw head may cause the head material to yield or otherwise deform when subjected to loading of the vertebra. Failure or deformation of the bone screw head may also compromise the stability of the bone plate system which, in turn, could cause the bone fusion procedure to fail. Another problem with these configurations is that the stress on the system may lead to the set screw backing out over time.
Other bone plate systems fix a bone screw within the bore of a bone plate using an annular bushing located within the bore before the bone screw is inserted therethrough. The bone screw is then inserted through an opening in the bushing until the bone screw head is positioned within the bushing. The bone screw head may have an outer profile that causes the bushing to expand into tight engagement with the bore surface. In prior bone plates, the movement of the bushing is limited to the confines of the bore in which the bushing is received. As is apparent, the angle at which the bone screw is inserted through the bore and into a bone is similarly limited by the limited movement of the bushing within the bone plate bore. Further, the insertion angle of the bone screw may also be limited by contact between the bone screw and the bone plate as the bone screw is driven into the bone plate bore. Specifically, the bone screw head may catch on the upper lip of the bone plate extending about the bore and resist passage into the bore, such that a different insertion angle may be required.
One problem with bushings that are placed into bone plate bores is the assembly time this requires. Another problem with this arrangement is that the surgeon has to align the bone screw with the opening in the bushing before passing the bone screw therethrough. This complicates the process of fastening the bone plate onto the bones and increases the duration of the surgical procedure. In addition, one approach as disclosed in U.S. Pat. No. 6,030,389 to Wagner et al. involves a separate step of screwing a set screw into an anchor head to expand the bone anchor head and fix the anchor relative to the bone plate. The set screw is necessarily larger than the bone anchor head in order to expand a bushing positioned within the bore. Not only does the large set screw require time-intensive installation, but it also extends a significant distance above the head of the bone anchor such that a thicker bone plate is required to limit the distance the set screw projects into the body.